Microwave rotatable joint



1953 J. F. ZALESKI MICROWAVE ROTATABLE JOINT Filed Sept. 20, 1951 Patented Oct. 6, 1953 MICROWAVE. ROTATDABLE. JOIN J clin F. Zaleski, Valhalla, N1 Y., assignor to Gen-- eral Precision. Laboratory Incorporated, a. corporation of. New York Application September 20, 195.1}, ScLialNo. 241,51

(Cl;v 333-33.)

10-Claims. 1

This invention pertains to microwave hollow guiderotatable joints and more specifically to rotata-ble joints which provide angularly variable connections between hollow guides of rectangular' or circular cross-section.

In interposing a rotating joint in rectangular guide some device is obviously necessary to control' the effective microwave polarization in the joint so that at each rectangular terminal the microwave polarization is exactly the-same as the terminal orientation. In other words, the microwave energy asi't passes through the device must be changed in field polarization by exactly the same amount as thedevice rotates in its physical orientation.

In interposing arotating joint in round guide any ordinary type of" joint can be used if the guide: carries: a circularly symmetrical mode, but if the guide carries the dominant or Til-1,1 mode, or any other non-symmetrical mode, a joint that forces the field to rotate. as the joint is. rotated is essential. This. action. occurs in the joint. of the present. invention.

The joint of the present. invention is. completelymobile; that is, itcan. be. rotated for any number of degrees at any speed which is me:- chemically feasible. The joint can be. employed for the. mobile coupling of two rectangular guides. two round guides, or of a rectangular guide tol'a round guide.

The invention. provides two similar guide transitions connected by a rotary joint. Each transition is of the type used for the. axial con-- nection of a, coaxial transmission line with a, rectangular or round hollow guide, and each transition includes a hollow guide; side arm stub and an antenna positioned within the hollow guide. The hollow guide sidearm stub is preferably, ior convenience, of the rectangular type, whether connected to a rectangular or round guide termination. The two transitions are connected by their coaxial members, which therefore, form a substantially singlecommon member, and which contains at a mid-position some modified type of coaxial rotary joint. Each half of the device is a complete transition from coaxial conductor to rectangular or round guide, and hasv utility in and of itself as a sub-combination which can be used elsewhere than in a complete rotatable joint.

The general purpose of this invention is to provide a compact and efiicient motional joint in, hollow waveguide which is easy to design and construct for use at any microwave length.

A specific purpose is toprovide a mobile joint for two rectangular guides which are axially aligned.

Another purpose; is to provide a compactrotaryjoint for: rectangular guide or round guide when operated in a, non-circular mode.

A; further purpose is to; provide. an, irm rovedv end-to-end transition between; arectangularhollow guide and: coaxial. microwave conductors.

A further understanding; of the invention may be secured trom; the; detailed description. and the; accompanying: drawings in which;

Fig. l isv an, oblique View of oneembodiment of the microwave guide rotatable joint of the invention.

Fig. 2, is a sectional view taken, on, the line 2--2 ofrFig. 1.

Fig. 3 issa sectionalview-taken. on the line 3.-3. of, Fig. 1.

Fig. 4 isa schematicview illustrating the, opera". tionof. the device.

Referring, now toEigl 1,. a rectangular guide ll is secured and. electrically connected to one end of, thcdevice-of the invention by meansof a con-. ventionalchoke. flange l2 fastened to a plain The. twoflan es. l2 and I3, are lastenedtogether by means. of four screws or which two are visible. at M and 16,. Such a choke flange coupling is efiicient and is well known in the art, so. will, not-be further described. A similar rectangular guide. I] is. connected by S milar cho e and plainfianges, [8, and [9, to. the other end of the. device.

Either oi the. two. rectan l r. uides I! an l canbe used to introduce.microwaveenergy to the, device, the: other guide thenbeing the. output, the deviceyis functionally bisymmetric i.

The. rectan ular guide. H. is, connected. through theflanges; l2 and I3.- and a short length of rectangular guide to, a coaxialtransmission line, 2 I a tuned side arm stub 22; being provided at the junction of the twotypes of transmission members to assist in matching or tuning out the impedance discontinuity caused by such a junction and for other reasons as will be further described. It is necessary only that this side arm should support a waveguide mode for the microwave, frequency which is to be employed, and

therefore may be a, short-circuited stub, which is closed at its upper end by a metallic plate.

The rectangular guide I1 is similarly connected to the other end of the same coaxial transmission line 2 I, employing a similar shortecircuited rectangular side arm stub 23.

The coaxial transm ssion line cons sts of a central conductor 24 (Fig. 2) and an outer conductor 26. The left end (as illustrated in Fig. 2) of the outer conductor 26 is mechanically secured to the side arm matching stub 22 and through it to the flange [3. However, electrically the rec tangular guide ll secured to the flange it can be considered as being prolonged to the cornmencement of the coaxial transmission line at the juncture 21. Similarly, the right end of the outer conductor 26 is mechanically connected to the side arm stub 23 and through it to the flange IS.

The coaxial transmission line 2! should be so dimensioned that only thefundamental TEM mode will be propagated therethrough at the 'frequency impressed thereon. As is well known by those skilled in the art the limiting or cutofi wavelength in free space at which the first higher mode will be propagated is given approximately by the expression:

where a is the diameter of the inner conductor and b is the internal diameter of the outer conduotor. Thus the dimensions a and I) must be such that 7x as calculated by the above expression is in excess of the wavelength in free space of the microwave energy impressed thereon. Likewise, the length of the coaxial line 2| lying between the rectangular wave guide inputs considered as the junction lines 21 and 28 should be at least a half wavelength in free space in length.

The center conductor 24 extends beyond the respective ends 21 and 28 of the coaxial transmission line into spaces which electrically are parts of the rectangular guides axially aligned with the coaxial transmission line. These extensions of the center conductor therefore constitute antennas extending axially into the guides. The extended antennas are tapered so that their ends 29 and 3! present relatively small areas to the rectangular guides and therefore produce only minor electrical discontinuities. However, non-tapered extensions could be used with minor changes in extension length to compensate for the impedance discontinuity introduced by the blunt ends.

The central conductor 24 is supported by two transverse rods 32 and 33 which extend from side to side of the rectangular guide and are secured in the narrower sides thereof. This is more clearly seen in Fig. 3. Since the two rods 32 and 33 are thin and are perpendicular at all points to the direction of the electric component of the field in the rectangular guide, these rods do not afiect the impedance of the rectangular guide in which they are placed. The end 3| of the cen-,

tral conductor is not secured in any way.

The outer coaxial conductor 26, Fig. 2, is annularly broken at 34 to permit rotation, this break being made a point of low impedance by a closed folded slot one-half wavelength long. The one-half wavelength slot is folded into two quarter-wave cylindrical slots 36 and 31 and the mechanical junction 38 at the outer surface is a point of high impedance because it is onequarter wavelength from the closed end of the slot. This design permits relative rotation of the two ends of the device without causing a discontinuity, sparking or radiation.

Numerous other designs of a wave-trap mechanical joint are known and may alternatively be used here. One such design, for example, consists simply of a coaxial transmission line having a rotatable choke flange joint in its outer conductor.

In operation, microwave energy is applied to one end or the other of the device through the attached rectangular guide, for example through a guide attached to the flange 13. The electrical component of the field is thus vertical as it might be observed in Fig. 2, as themicrowave energy is applied to the device. The energy flows into the side arm stub 22, the length of which is made to be an odd integral multiple of a quarter wavelength in guide, so that standing waves are generated with a voltage maximum at the horizontal axial plane of the incoming rectangular guide.

This is more easily visualized in Fig. 4, in which the electric component of the incoming energy is represented in the guide [I by equally-spaced vertical lines 39. These lines bend around the corner 4|, as indicated by the oblique line 42. The line 43 represents the center line of the axially-placed antenna, and the closed end 44 of the side arm stub 22 being, for example, onequarter wavelength from the antenna, a standing voltage loop is placed at the position of the antenna 43 with the direction of the concentrated electrical field along the direction of the antenna. This is the most favorable condition for coupling of the electrical field to the antenna, with the result that there is maximum coupling of the incoming field energy to the antenna 43, and the latter may therefore receive and extract substantially all of the energy from the incoming microwave field.

Viewed from the input rectangular guide H, the presence of the side arm 22 and of the antenna both cause electrical discontinuities. However, their effects are completely matched out by proper design of the length L of the side arm and the distance D by which the antenna 4 extends into the rectangular guide space. Such dimensions are customarily determined empirically in microwave transmission apparatus, only rough and inaccurate calculations being possible in most cases. As an example, however, it has been found that where microwave energy having a frequency of 8800 megacycles is used, the length L may be 0.6 inch and the distance D 0.5 inch, when the coaxial line inner conductor diameter is .187, and the outer conductor inside diameter is .437".

The antenna 43 is merely a prolongationv of the center conductor of the coaxial transmission line, the taper of the antenna as illustrated in Fig. 2 being desirable to permit employment of the best thickness of coaxial center conductor while presenting only a small area of the antenna end to the incoming energy. The described excitation of the antenna is most favorable for setting up the TEM mode of field about the coaxial center conductor, and the input energy is therefore transferred completely to the coaxial transmission line. At the output end, the construction being similar to that at the input end, the energy in the TEM field of the coaxial line is reconverted to TE1,0 field mode energy in the output rectangular guide by a process which is the reverse of that described.

Since the microwave energy in the coaxial portion of the device is in the circularly symmetrical TEM mode, no electrical change is produced by mechanical rotation of one end of the device in relation to the other end. As the central conductor 24 is secured to one end only of the device, it rotates as a whole with that end of the device and no joint is required in it.

What is claimed is:

1. A microwave rotatable joint comprising, a pair of hollow guides in axial alignment, a co axial line containing a coaxial rotary joint, first transition means connecting one of said hollow guides to one end of said coaxial line in axial alignment therewith, said first transition means including a transverse hollow guide matching stub, and second transition means connecting the other of said hollow guides to the other end of said coaxial line in axial alignment therewith, said second transition means including a transverse hollow guide matching stub.

2. A microwave rotatable joint comprising, a pair of hollow guides in axial alignment, a coaxial line containing a coaxial rotary joint, first transition means axially connecting one of said hollow guides to one end of said coaxial line in axial alignment therewith, said first transition means including a transverse hollow guide tuning stub and an antenna constituting an extension of the inner conductor of said coaxial line, and second transition means axially connecting the other of said hollow guides to the other end of said coaxial line in axial alignment therewith, said second transition means including a transverse hollow guide tuning stub and an antenna constituting an extension of the inner conductor of said coaxial line.

3. A microwave rotatable joint comprising, a pair of hollow guides in axial alignment, a coaxial line containing a coaxial rotary joint, first transition means axially connecting one of said hollow guides to one end of said coaxial line in axial alignment therewith, said first transition means including a hollow guide tuning stub perpendicular to the axis of the first hollow guide and an antenna prolongation of the inner conductor of said coaxial line, and second transition means axially connecting the other of said hollow guides to the other end of said coaxial line in axial alignment therewith, said second transition means including a hollow guide tuning stub perpendicular to the axis of the second hollow guide and an antenna prolongation of the inner conductor of said coaxial line.

4. A microwave rotatable joint comprising, a pair of hollow guides positioned with their axes in a single axial line, a coaxial line containing a coaxial rotary joint, the axis of which lies in said axial line, first transition means connecting one of said hollow guides to one end of said coaxial line with the axis of said one hollow guide lying in said axial line, said first transition means inincluding a hollow guide short-circuited tuning stub having a length substantially equal to an odd integral multiple of a selected quarter microwave length and an antenna formed by a prolongation of one end of the inner conductor of said coaxial line, and second transition means connecting the other of said hollow guides to the other end of said coaxial line with the axis of said other hollow guide in said axial line, said second transition means including a hollow guide short-circuited tuning stub having a length substantially equal to an odd integral multiple of said selected quarter microwave length and an antenna formed by a prolongation of the other end 01 said inner conductor of the coaxial line.

5. A microwave rotatable joint comprising, a pair of hollow guides positioned with their axes in a single axial line, a coaxial line containing a coaxial rotary joint and having its axis lying in said axial line, a first transition connected between one of said hollow guides and one end of said coaxial line, said first transition including a hollow guide short-circuited tuning stub having a length substantially equal to an odd integral multiple of a selected quarter-microwave length, said stub being perpendicular to said axial line, and including a first antenna prolongation of one end of the inner conductor of said coaxial line, and a second transition connected between the other of said hollow guides and the other end of said coaxial line, said second transition including a hollow guide short-circuited tuning stub having a length substantially equal to an odd integral multiple of said selected quarter-microwave length, said stub being perpendicular to said axial line, and including a second antenna prolongation of the other end of the inner conductor of said coaxial line.

6. A microwave device in accordance with claim 5 in which said hollow guides are rectangular in cross section.

'7. A microwave device in accordance with claim 6 in which said inner conductor is anchored by at least one transverse pin embedded at its ends in the narrow side walls of one of said rectangular hollow guides, said pin being perpendicular to said narrow side walls.

8. A microave device in accordance with claim 5 in which said coaxial rotary joint is in the outer conductor only of said coaxial line and in which the inner conductor of said coaxial line is anchored at one end only.

9. A microwave transition for the in-line connection of a hollow guide to a coaxial line comprising, a tuned rectangular guide stub connected between the end of the hollow guide and the end of the coaxial line positioned with its axis perpendicular to the hollow guide, and an antenna positioned in said guide stub coaxially with and connected to the inner conductor of said coaxial line.

10. A microwave transition in accordance with claim 9 in which said guide stub is provided with a short circuit end portion and has a length equal to an odd integral multiple of one quarter of the wavelength of the microwave energy selected for energization.

JOHN F. ZALESKI.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,292,496 Von Baeyer Aug. 11, 1942 2,433,011 Zaleski Dec. 23, 1947 2,434,509 Okress Jan. 13, 1948 2,451,876 Salisbury Oct. 19, 1948 2,501,335 Hunter Mar. 21, 1950 2,526,678 Mallett Oct. 24, 1950 2,627,551 Taylor Feb. 3, 1953 

